Hydrogel-forming composition and hydrogel produced from the same

ABSTRACT

TASK It is an object of the present invention to provide a hydrogel having excellent mechanical properties and capable of being produced simply by using and mixing an industrially easily obtainable polymer having high versatility and clay particles, and to provide a method of producing the hydrogel. 
     MEANS OF SOLVING THE PROBLEM A hydrogel-forming composition is characterized by containing a polyelectrolyte (A), clay particles (B), and a dispersant (C) for the clay particles.

TECHNICAL FIELD

The present invention relates to a hydrogel. More in detail, the presentinvention relates to a polymer/nano fine particle compositehydrogel-forming composition and a polymer/nano fine particle compositehydrogel produced from the composition and having excellent mechanicalproperties.

BACKGROUND ART

A polymer hydrogel contains water as a main component, so that it ishighly safe and it can be inexpensively produced. The polymer hydrogelplaces a low burden on the environment, so that it is attractingattention as a material for a soft material and it is utilized for anaromatic, a jelly, a paper diaper, and the like.

However, many types of polymer hydrogels have a heterogeneous structurein which the crosslinkage density has a distribution, and they aremechanically brittle.

To compensate for such a drawback on the mechanical properties, apolymer/nano fine particle composite-type hydrogel has been attractingattention.

As a polymer/nano fine particle composite hydrogel having excellentmechanical properties, there is disclosed a nano composite gel obtainedby radically polymerizing an acrylamide-based monomer in the presence ofdelaminated clay particles in water (Patent Document 1 and Non-patentDocument 1). As an example of a similar disclosure, there is also knowna nano composite gel composed of a polymer containing a group having acarboxylic acid salt structure or a carboxylic acid anion structure insome part of the poly(meth)acrylamide and clay particles (PatentDocument 2). Furthermore, there is disclosed a hydrogel obtained bymixing sodium polyacrylate, clay particles, and a polyion dendrimerhaving a cationic functional group at a terminal thereof (PatentDocument 3 and Non-patent Document 2).

On the other hand, a dry clay film composed of a polyacrylic acid saltthat is a commonly-used polymer and clay is known and is studied as asurface protecting material (Patent Document 4).

A study on a viscosity change of an aqueous dispersion of sodiumpolyacrylate and clay is known (Non-patent Document 3). This is not astudy on a hydrogel having excellent mechanical properties.

RELATED-ART DOCUMENT Patent Documents

-   Patent Document 1: Japanese Patent Application Publication No.    2002-053629 (JP 2002-053629 A)-   Patent Document 2: Japanese Patent Application Publication No.    2009-270048 (JP 2009-270048 A)-   Patent Document 3: International Publication No. WO 2011/001657-   Patent Document 4: Japanese Patent Application Publication No.    2009-274924 (JP 2009-274924 A)

Non-Patent Documents

-   Non-patent Document 1: K. Haraguchi, et. al., Adv. Mater., 14(16),    1120 (2002)-   Non-patent Document 2: T. Aida, et. al., Nature, 463 339 (2010)-   Non-patent Document 3: Colloids and Surfaces, A Physicochemical and    Engineering Aspects (2007), 301(1-3), 8-15

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In Patent Document 1 to Patent Document 3, Non-patent Document 1, andNon-patent Document 2, polymer/nano fine particle composite gels havingexcellent mechanical properties are disclosed.

However, the polymer/nano fine particle composite gels described inPatent Document 1, Patent Document 2, and Non-patent Document 1 requirea polymerization reaction in a production process thereof. The hydrogelsdisclosed in Patent Document 3 and Non-patent Document 2 require the useof special polymers (polyion dendrimers) in the production processesthereof.

In Patent Document 4, although as an intermediate, a gel paste isproduced, the gel paste is mechanically brittle. The gel paste isapplied onto a sheet and the resultant film after drying of the gelpaste has excellent mechanical properties.

Therefore, there is desired a polymer/nano fine particle compositehydrogel having excellent mechanical properties and capable of beingproduced by a simpler and commonly-used method using a polymer havinghigh versatility.

Thus, the present invention has been invented according to the abovesituation and it is an object of the present invention to provide apolymer/nano fine particle composite hydrogel having excellentmechanical properties and capable of being produced simply by using andmixing an industrially easily obtainable polymer having high versatilityand a clay mineral (expressed also as clay particles in the presentspecification).

Means for Solving the Problem

As a result of assiduous research for solving the above problem, theinventors of the present invention found that by mixing apolyelectrolyte, clay particles, and a dispersant for the clayparticles, a polymer/nano fine particle composite hydrogel havingexcellent mechanical properties can be obtained, and have completed thepresent invention.

That is, the present invention relates to, according to a first aspect,a hydrogel-forming composition, characterized by containing apolyelectrolyte (A), clay particles (B), and a dispersant (C) for theclay particles.

The present invention relates to, according to a second aspect, thehydrogel-forming composition according to the first aspect, in which thepolyelectrolyte (A) is a polyelectrolyte having an organic acid saltstructure or an organic acid anion structure.

The present invention relates to, according to a third aspect, thehydrogel-forming composition according to the second aspect, in whichthe polyelectrolyte (A) is a polyelectrolyte having a carboxylic acidsalt structure or a carboxylic acid anion structure.

The present invention relates to, according to a fourth aspect, thehydrogel-forming composition according to the third aspect, in which thepolyelectrolyte (A) is a completely neutralized or partially neutralizedpolyacrylic acid salt.

The present invention relates to, according to a fifth aspect, thehydrogel-forming composition according to the fourth aspect, in whichthe polyelectrolyte (A) is a completely neutralized or partiallyneutralized polyacrylic acid salt having a weight average molecularweight of 1,000,000 to 10,000,000.

The present invention relates to, according to a sixth aspect, thehydrogel-forming composition according to any one of the first aspect tothe fifth aspect, in which each of the clay particles (B) is awater-swelling silicic acid salt particle.

The present invention relates to, according to a seventh aspect, thehydrogel-forming composition according to the sixth aspect, in whicheach of the clay particles (B) is a water-swelling silicic acid saltparticle selected from the group consisting of smectite, bentonite,vermiculite, and mica.

The present invention relates to, according to an eighth aspect, thehydrogel-forming composition according to any one of the first aspect tothe seventh aspect, in which the dispersant (C) is a dispersant forwater-swelling silicic acid salt particles.

The present invention relates to, according to a ninth aspect, thehydrogel-forming composition according to the eighth aspect, in whichthe dispersant (C) is one or two or more selected from the groupconsisting of sodium orthophosphate, sodium diphosphate, sodiumtripolyphosphate, sodium tetraphosphate, sodium hexametaphosphate, andsodium polyphosphate.

The present invention relates to, according to a tenth aspect, ahydrogel produced from the hydrogel-forming composition described in anyone of the first aspect to the ninth aspect.

The present invention relates to, according to an eleventh aspect, amethod of producing a hydrogel, characterized by mixing thepolyelectrolyte (A), the clay particles (B), and the dispersant (C)defined in any one of the first aspect to the ninth aspect, and water ora water-containing solvent to cause gelation of the resultant mixture.

Effects of the Invention

The hydrogel of the present invention has excellent mechanicalproperties, that is, has satisfactory strength. For example, typically,the hydrogel has such rigidity (“elastic modulus”) or such strength(“breaking stress”) that allows the hydrogel to maintain the shape ofthe gel without a support such as a container, that is, has so-calledself-supporting properties.

The hydrogel of the present invention can be produced by mixing thepolyelectrolyte, the clay particles, the dispersant, and water or awater-containing solvent.

Furthermore, the elastic modulus of the hydrogel of the presentinvention can be controlled by varying the content of the component (B):clay particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the results of degrees of swelling measured inExample 28.

FIG. 2 is a photograph showing hydrogels at time points A, B, and Cspecified in FIG. 1.

FIG. 3 is a graph of the results of stress changes measured in Example29.

FIG. 4 is a graph showing a relation between the concentration of theclay particle and the elastic modulus that is obtained from FIG. 3.

FIG. 5 is a graph of the results of stress changes measured in Example30.

FIG. 6 is a graph showing a relation between the concentration of thepolyelectrolyte and the elastic modulus that is obtained from FIG. 5.

FIG. 7 is a graph showing a relation between the concentration of adispersant and the elastic modulus.

FIG. 8 is a graph showing a relation between the concentration of adispersant and the elastic modulus

FIG. 9 is a graph showing a relation between the concentration of adispersant and the elastic modulus.

FIG. 10 is a photograph showing hydrogels of respective compositions ofD and E in FIG. 9 after a uniaxial compression test.

FIG. 11 is a table of the results of a small angle X-ray scattering(SAXS) measurement under extension.

MODES FOR CARRYING OUT THE INVENTION

The clay particle, for example, LAPONITE (manufactured by RockwoodAdditives Limited, registered trade name of Rockwood Additives Limited)is a disk-shaped particle in which: an edge face is positively chargedand, on the other hand, a surface is negatively charged; and a layerstructure is formed through a sodium ion. By adding water to thisLAPONITE particle, a sodium ion is hydrated with a water molecule and adelamination is caused. Then, a positively-charged edge face of aLAPONITE particle is bonded with a negatively-charged surface of aLAPONITE particle through an electrostatic interaction and it is knownthat as the result thereof, the LAPONITE particles form a card housestructure and enhance the viscosity of an aqueous dispersion thereof.

On the contrary, it is also known that a dispersant such as sodiumdiphosphate (alias: sodium pyrophosphate) is adsorbed to the positivelycharged edge face of the LAPONITE particle to accelerate the dispersionof the LAPONITE particle and further to suppress the formation of thecard house structure.

It is considered that when a polyelectrolyte is added into a state inwhich clay particles are homogeneously dispersed in water, aninteraction between the clay particle and the polyelectrolyte is newlygenerated and a homogeneous polymer/clay network is constructed, so thata polymer/nano fine particle composite hydrogel having excellentmechanical properties is formed.

That is, the present invention is based on the findings that by blendinga dispersant (C) and a polyelectrolyte (A) to clay particles (B), apolymer/nano fine particle composite hydrogel excellent in mechanicalproperties can be obtained, and relates to a hydrogel-formingcomposition characterized by containing a polyelectrolyte (A), clayparticles (B), and a dispersant (C).

In the hydrogel-forming composition of the present invention and ahydrogel produced therefrom, besides the components (A) to (C), ifnecessary, other components may be optionally blended so long as theexpected effect of the present invention is not impaired.

[Hydrogel-Forming Composition]

<Component (A): Polyelectrolyte>

The component (A) of the present invention is a polyelectrolyte,preferably an anionic polyelectrolyte, more preferably a polyelectrolytehaving an organic acid salt structure or an organic acid anionstructure.

Examples of such a polyelectrolyte include: a polyelectrolyte having acarboxy group such as a poly(meth)acrylic acid salt, a salt of acarboxy-vinyl polymer, and a salt of carboxy methylcellulose; apolyelectrolyte having a sulfonyl group such as a polystyrenesulfonicacid salt; and a polyelectrolyte having a phosphonyl group such as apolyvinyl phosphonic acid salt. Examples of the salt include a sodiumsalt, an ammonium salt, a potassium salt, and a lithium salt. In thepresent invention, (meth)acrylic acid refers to both acrylic acid andmethacrylic acid.

The polyelectrolyte (A) may be crosslinked or copolymerized and may be acompletely neutralized product or a partially neutralized product.

The polyelectrolyte (A) has a weight average molecular weight measuredby gel permeation chromatography (GPC) in terms of polyethylene glycolof preferably 1,000,000 to 10,000,000, more preferably 2,000,000 to7,000,000.

A commercially available polyelectrolyte (A) has a weight averagemolecular weight described on the commercial product of preferably1,000,000 to 10,000,000, more preferably 2,000,000 to 7,000,000.

In the present invention, among the above polyelectrolytes, thepolyelectrolyte (A) is preferably a polyelectrolyte having a carboxylicacid salt structure or a polyelectrolyte having a carboxylic acid anionstructure, particularly preferably a completely neutralized or partiallyneutralized polyacrylic acid salt. Specifically, the polyelectrolyte (A)is preferably sodium polyacrylate, particularly preferably anon-crosslinked-type highly polymerized sodium polyacrylate having aweight average molecular weight of 2,000,000 to 7,000,000.

The content of the polyelectrolyte (A) is 0.01% by mass to 20% by massand preferably 0.1% by mass to 10% by mass relative to 100% by mass ofthe hydrogel.

In the present specification, % by mass is expressed also as % byweight.

<Component (B): Clay Particle>

The component (B) of the present invention is a clay particle,preferably a clay nano fine particle. Specifically, the component (B) ispreferably a water-swelling silicic acid salt particle.

Examples of the clay particles (B) include smectite, bentonite,vermiculite, and mica and the clay particles (B) are preferablyparticles forming colloid in a dispersion medium of water or awater-containing solvent. Examples of the shape of a primary particle ofthe clay include a disk shape, a plate shape, a sphere shape, a grainshape, a cube shape, a needle shape, a bar shape, and an amorphousshape, and the shape is preferably a disk shape or plate shape having adiameter of 5 nm to 1,000 nm.

Preferred specific examples of the clay include a layered silicic acidsalt and easily commercially available examples thereof include: theproducts manufactured by Rockwood Additives Limited such as LAPONITE XLG(synthetic hectorite), XLS (synthetic hectorite, containing sodiumdiphosphate as a dispersant), XL21 (sodium magnesium fluorosilicate), RD(synthetic hectorite), RDS (synthetic hectorite, containing inorganicpolyphosphoric acid salt as a dispersant), and S482 (synthetichectorite, containing a dispersant); the products manufactured by Co-opChemical Co., Ltd. such as Lucentite (registered trade name of Co-opChemical Co., Ltd.) SWN (synthetic smectite) and SWF (syntheticsmectite), micro mica (synthetic mica), and Somasif (registered tradename of Co-op Chemical Co., Ltd., synthetic mica); the productsmanufactured by Kunimine Industries Co., Ltd. such as Kunipia(registered trade name of Kunimine Industries Co., Ltd.,montmorillonite) and Sumecton (registered trade name of KunimineIndustries Co., Ltd.) SA (synthetic saponite); and the productsmanufactured by Hojun Co., Ltd. such as BEN-GEL (registered trade nameof Hojun Co., Ltd., purified product of natural bentonite).

The content of the clay particles (B) is 0.01% by mass to 20% by massand preferably 0.1% by mass to 15% by mass relative to 100% by mass ofthe hydrogel.

<Component (C): Dispersant for Clay Particle>

The component (C) of the present invention is a dispersant for the clayparticles (B) and is preferably a dispersant for water-swelling silicicacid salt particles.

As the dispersant (C), a dispersant or a deflocculant used for thepurpose of enhancing the dispersibility of the silicic acid salt ordelaminating a layered silicic acid salt can be used.

Examples of the dispersant (C) include sodium orthophosphate, sodiumdiphosphate (sodium pyrophosphate), sodium tripolyphosphate, sodiumtetraphosphate, sodium hexametaphosphate, and sodium polyphosphate.Among them, preferred is sodium diphosphate.

The content of the dispersant (C) is 0.001% by mass to 20% by mass andpreferably 0.01% by mass to 10% by mass, relative to 100% by mass of thehydrogel.

In the present invention, when, as the component (B), clay particlescontaining a dispersant are used, a dispersant as the component (C) maybe either further added or not added.

Examples of the preferred combination of the polyelectrolyte (A), theclay particles (B), and the dispersant (C) include a combination of apolyelectrolyte (A): 0.1% by mass to 10% by mass of sodium polyacrylatehaving a weight average molecular weight of 2,000,000 to 7,000,000, acomponent (B): 0.1% by mass to 15% by mass of water-swelling smectite orsaponite, and a component (C): 0.01% by mass to 10% by mass of sodiumdiphosphate, relative to 100% by mass of the hydrogel.

The hydrogel-forming composition of the present invention may contain analcohol.

The alcohol is preferably a water-soluble alcohol capable of beingfreely dissolved in water, more preferably a C₁₋₈ alcohol and specificexamples thereof include methanol, ethanol, 2-propanol, isobutanol,pentanol, hexanol, 1-octanol, and isooctanol.

[Hydrogel and Production Method Thereof]

The hydrogel of the present invention can be obtained by gelation of thehydrogel-forming composition.

The gelation using the hydrogel-forming composition can be performed bymixing a mixture of two components of the hydrogel-forming composition,an aqueous solution thereof, or an aqueous dispersion thereof and therest one component, an aqueous solution thereof, or an aqueousdispersion thereof. The gelation can also be performed by adding waterto a mixture of these components.

Although as the method for mixing the components in the hydrogel-formingcomposition, an ultrasonication can be used besides a mechanicalstirring and a manual stirring, a mechanical stirring is preferred.Examples of the machine capable of being used for the mechanicalstirring include a magnetic stirrer, a propeller stirrer, arotation-revolution-type mixer, a dispenser, a homogenizer, a shaker, avortex mixer, a ball mill, a kneader, and a ultrasonic wave oscillator.

The temperature for the mixing is a solidifying point to a boiling pointof the above aqueous solution or the above aqueous dispersion,preferably −5° C. to 100° C. and more preferably 0° C. to 50° C.

When foam is generated immediately after the mixing, defoaming ispreferably performed using a centrifuge. The time for defoaming using acentrifuge is, for example, 10 minutes to 20 minutes.

Although the mixture immediately after the mixing is a sol having asmall strength, when the mixture is settled, the mixture becomes gelled.The time for settling is preferably 2 hours to 100 hours. Thetemperature for settling is −5° C. to 100° C. and preferably 0° C. to50° C. By pouring the mixture before gelled immediately after the mixinginto a mold or by extrusion-molding the mixture, a gel can be producedin any shape.

The strength of the obtained hydrogel can be measured in a uniaxialcompression test. For example, a hydrogel in a column shape having adiameter of 10 mm and a height of 6 mm is produced and the hydrogel issettled at room temperature for 4 days. Then, the strength of thehydrogel can be measured using TENSILE TESTER STM-20 (manufactured byOrientec Co., Ltd.). The measuring method is a method by compressing themolded sample at a compression rate of 10 mm/min to measure a stresschange.

The stress at a time point when the distortion factor is 80% (ΔL/L₀=0.8)is measured by a measurement in the above uniaxial compression test.Here, ΔL is a compressed length and L₀ is a natural length. The stressis measured by taking into consideration an area change (calculatedunder such an assumption that the volume is not changed during thecompression).

The stress of the hydrogels obtained by the present invention at a timepoint when the distortion factor is 80% (ΔI/L₀=0.8) is 10 kPa to 1,000kPa. For an application of the hydrogel requiring high strength,examples of the lower limit of the stress include 12 kPa, 20 kPa, and 50kPa and examples of the upper limit of the stress include 150 kPa, 300kPa, and 500 kPa. Examples of the stress ranges are 12 kPa to 150 kPaand 50 kPa to 500 kPa.

EXAMPLES

Hereinafter, the present invention will be more specifically describedreferring to Examples that should not be construed as limiting the scopeof the present invention.

Example 1 Production of LAPONITE RD 5% by Weight/ASAP 1% by Weight/TSPP0.5% by Weight Hydrogel

0.021 g of sodium diphosphate decahydrate (sodium pyrophosphatedecahydrate) (TSPP) (manufactured by Kanto Chemical Industry Co., Ltd.)and 2.328 g of water were mixed and the resultant mixture was stirredwith a magnetic stirrer at room temperature (about 20° C.) until themixture became a homogeneous TSPP aqueous solution. Then, while the TSPPaqueous solution was stirred with a magnetic stirrer, to the TSPPaqueous solution, 0.126 g of LAPONITE RD (manufactured by RockwoodAdditives Limited) was added little by little and the resultant mixturewas stirred at room temperature (about 20° C.) until the mixture becamea homogeneous aqueous dispersion. Then, to the resultant aqueousdispersion, 0.026 g of sodium polyacrylate (ASAP) (manufactured by WakoPure Chemical Industries, Ltd.; degree of polymerization: 22,000 to70,000, viscosity of a 2 g/L aqueous solution thereof at 30° C.: 350 to560 mPa·s) was added little by little and the resultant mixture wasstirred manually using a spatula instead of the magnetic stirrer for 5minutes to 30 minutes.

Then, the mixture was defoamed using a centrifuge (at 7,000 rpm for 10minutes) and was settled at room temperature for 48 hours to obtain ahydrogel.

Example 2 to Example 5 Production of LAPONITE RD 7.5% by Weight to 15%by Weight/ASAP 1% by Weight/TSPP 0.5% by Weight Hydrogels

By the same operation as in Example 1, a hydrogel which containsLAPONITE RD at each concentration listed in Table 1 was produced.

TABLE 1 LAPONITE RD concentration ASAP concentration TSPP concentrationExample [% by weight] [% by weight] [% by weight] 1 5 1 0.5 2 7.5 1 0.53 10 1 0.5 4 12.5 1 0.5 5 15 1 0.5

Example 6 Production of LAPONITE RD 10% by Weight/ASAP 0.25% byweight/TSPP 0.5% by Weight Hydrogel

0.017 g of sodium diphosphate decahydrate (sodium pyrophosphatedecahydrate) (TSPP) (manufactured by Kanto Chemical Industry Co., Ltd.)and 1.772 g of water were mixed and the resultant mixture was stirredwith a magnetic stirrer at room temperature (about 20° C.) until themixture became a homogeneous TSPP aqueous solution. Then, while the TSPPaqueous solution was stirred with a magnetic stirrer, to the TSPPaqueous solution, 0.200 g of LAPONITE RD (manufactured by RockwoodAdditives Limited) was added little by little and the resultant mixturewas stirred at room temperature (about 20° C.) until the mixture becamea homogeneous aqueous dispersion. Then, to the resultant aqueousdispersion, 0.005 g of sodium polyacrylate (ASAP) (manufactured by WakoPure Chemical Industries, Ltd.; degree of polymerization: 22,000 to70,000, viscosity of a 2 g/L aqueous solution thereof at 30° C.: 350 to560 mPa·s) was added little by little and the resultant mixture wasstirred manually using a spatula instead of the magnetic stirrer for 5minutes to 30 minutes.

Then, the mixture was defoamed using a centrifuge (at 7,000 rpm for 10minutes) and was settled at room temperature for 48 hours to obtain ahydrogel.

Example 7 to Example 12 Production of LAPONITE RD 10% by weight/ASAP0.5% by Weight to 5% by Weight/TSPP 0.5% by Weight Hydrogels

By the same operation as in Example 6, a hydrogel which contains ASAP ateach concentration listed in Table 2 was produced.

TABLE 2 LAPONITE RD concentration ASAP concentration TSPP concentrationExample [% by weight] [% by weight] [% by weight] 6 10 0.25 0.5 7 10 0.50.5 8 10 1 0.5 9 10 2 0.5 10 10 3 0.5 11 10 4 0.5 12 10 5 0.5

Example 13 Production of LAPONITE RD 5% by Weight/ASAP 1% by Weight/TSPP0.25% by Weight Hydrogel

0.0084 g of sodium diphosphate decahydrate (sodium pyrophosphatedecahydrate) (TSPP) (manufactured by Kanto Chemical Industry Co., Ltd.)and 1.8727 g of water were mixed and the resultant mixture was stirredwith a magnetic stirrer at room temperature (about 20° C.) until themixture became a homogeneous TSPP aqueous solution. Then, while the TSPPaqueous solution was stirred with a magnetic stirrer, to the TSPPaqueous solution, 0.100 g of LAPONITE RD (manufactured by RockwoodAdditives Limited) was added little by little and the resultant mixturewas stirred at room temperature (about 20° C.) until the mixture becamea homogeneous aqueous dispersion. Then, to the resultant aqueousdispersion, 0.02 g of sodium polyacrylate (ASAP) (manufactured by WakoPure Chemical Industries, Ltd.; degree of polymerization: 22,000 to70,000, viscosity of a 2 g/L aqueous solution thereof at 30° C.: 350 to560 mPa·s) was added little by little and the resultant mixture wasstirred manually using a spatula instead of the magnetic stirrer for 5minutes to 30 minutes.

Then, the mixture was defoamed using a centrifuge (at 7,000 rpm for 10minutes) and was settled at room temperature for 48 hours to obtain ahydrogel.

Example 14, Example 15, and Comparative Example 1 Production of LAPONITERD 5% by Weight/ASAP 1% by Weight/TSPP 0.0% by Weight to 0.75% by WeightHydrogels

By the same operation as in Example 13, a hydrogel which contains TSPPat each concentration listed in Table 3 was produced. As a comparativeexample, a hydrogel containing no TSPP was produced (Comparative Example1).

TABLE 3 LAPONITE RD ASAP concentration concentration TSPP concentration[% by weight] [% by weight] [% by weight] Comparative 5 1 0.0 Example 1Example 13 5 1 0.25 Example 14 5 1 0.5 Example 15 5 1 0.75

Example 16 to Example 19 and Comparative Example 2 Production ofLAPONITE RD 10% by Weight/ASAP 1% by Weight/TSPP 0.0% by Weight to 1.0%by Weight Hydrogels

By the same operation as in Example 13, a hydrogel which contains TSPPat each concentration listed in Table 4, LAPONITE RD at a concentrationof 10% by weight, and ASAP at a concentration of 1% by weight wasproduced. As a comparative example, a hydrogel containing no TSPP wasproduced (Comparative Example 2).

TABLE 4 LAPONITE RD ASAP concentration concentration TSPP concentration[% by weight] [% by weight] [% by weight] Comparative 10 1 0.0 Example 2Example 16 10 1 0.25 Example 17 10 1 0.5 Example 18 10 1 0.75 Example 1910 1 1.0

Example 20 to Example 24 and Comparative Example 3 Production ofLAPONITE RD 15% by Weight/ASAP 1% by Weight/TSPP 0.0% by Weight to 1.25%by Weight Hydrogels

By the same operation as in Example 13, a hydrogel which contains TSPPat each concentration listed in Table 5, LAPONITE RD at a concentrationof 15% by weight, and ASAP at a concentration of 1% by weight wasproduced. As a comparative example, a hydrogel containing no TSPP wasproduced (Comparative Example 3).

TABLE 5 LAPONITE RD ASAP concentration concentration TSPP concentration[% by weight] [% by weight] [% by weight] Comparative 15 1 0.0 Example 3Example 20 15 1 0.25 Example 21 15 1 0.5 Example 22 15 1 0.75 Example 2315 1 1.0 Example 24 15 1 1.25

Example 25 to Example 27 Production of LAPONITE XLG 10% by Weight/TSPP0.5% by Weight Hydrogels Using 1% by Weight of ASAP Having CorrespondingMolecular Weight

By the same operation as in Example 3, except that LAPONITE RD waschanged to LAPONITE XLG (manufactured by Rockwood Additives Limited) andsodium polyacrylate (ASAP) (degree of polymerization: 22,000 to 70,000)was changed to sodium polyacrylate (ASAP) (manufactured by Wako PureChemical Industries, Ltd.; degree of polymerization: 2,700 to 7,500,viscosity of a 100 g/L aqueous solution thereof at 25° C.: 75 to 125mPa·s), sodium polyacrylate (ASAP) (trade name: Aronvis MX; manufacturedby Toagosei Co., Ltd.; weight average molecular weight: 2,000,000 to3,000,000), or sodium polyacrylate (ASAP) (trade name: Aronvis SX;manufactured by Toagosei Co., Ltd.; weight average molecular weight:4,000,000 to 5,000,000), each hydrogel was produced.

TABLE 6 ASAP molecular weight Example 25 250,000 to 710,000^((remark 1))Example 26 Weight average molecular weight 2,000,000 to 3,000,000Example 27 Weight average molecular weight 4,000,000 to 5,000,000^((remark 1))Degree of polymerization × molar mass of monomer unit(molar mass of monomer unit = 94)

Example 28 Measurements of Degree of Swelling and Gel Fraction ofHydrogel Using LAPONITE RD at Each Concentration

The degree of swelling was defined as W_(gel) (t)/W_(dry) and the gelfraction was defined as W_(dry)/W_(cal)×100 to measure the degree ofswelling and the gel fraction of the hydrogels produced in Example 1,Example 3, and Example 5.

Here, W_(gel) (t) is a mass of the sample after t hours from theinitiation of swelling; W_(dry) is a mass of the sample after theswelled sample was freeze-dried; and W_(cal) is a mass of the solute(ASAP+LAPONITE+TSPP) before the initiation of swelling, which iscalculated from a charged composition. The sample was defoamed by acentrifuge (at 7,000 rpm for 10 minutes) and was molded into a columnshape having a diameter of 6 mm and a length of 8 mm.

The molded sample was immersed in 200 mL of distilled water and wasretrieved after a lapse of a predetermined time and excessive waterdrops attached to the sample was wiped off to measure the mass (W_(gel)(t)) of the sample with a balance. After the sample reached anequilibrium swelling state, the sample was freeze-dried to measure themass (W_(dry)) of the sample. The results of the measurement of thedegree of swelling are listed in FIG. 1. FIG. 2 shows hydrogels at timepoints of A, B, and C specified in FIG. 1. As the result thereof, thegel fraction was 65% to 80%. The hydrogel of Example 1 in which theconcentration of LAPONITE RD was 5% by weight, the concentration of ASAPwas 1% by weight, and the concentration of TSPP was 0.5% by weight,exhibited maximum 1,200 times of the degree of swelling.

Example 29 Uniaxial Compression Test of Hydrogel Using LAPONITE RD atEach Concentration

The stress change of each of the hydrogels produced in Example 1 toExample 5 and Comparative Example 1 to Comparative Example 3 wasmeasured using TENSILE TESTER STM-20 (manufactured by Orientec Co.,Ltd.). The sample was defoamed using a centrifuge (at 7,000 rpm for 10minutes), was molded into a column shape having a diameter of 10 mm anda height of 6 mm, and was settled at room temperature for 4 days.

The molded sample was compressed at a compression rate of 10 mm/min tomeasure a stress change. According to a relational formula:σ=Eε(ε=ΔL/L₀) and from an inclination of a stress σ-distortion ε curvein a region in which the compression ratio is small, the elastic modulusE was calculated. Here, ΔL is a compressed length and L₀ is a naturallength. The stress was measured by taking into consideration an areachange by the compression (calculated under such an assumption that thevolume is not changed during the compression). The results of measuringthe stress change are listed in FIG. 3. The stress values at a timepoint when the distortion factor was 80% (ΔL/L₀=0.8) are listed in Table7. As the result thereof, by adding the dispersant (TSPP), the hydrogelexhibited a high stress value. In Comparative Example 1 to ComparativeExample 3, the gel collapsed largely at a time point when the distortionfactor was 80%. Furthermore, a relation between the LAPONITEconcentration and the elastic modulus is shown in FIG. 4. As the resultthereof, the higher the LAPONITE concentration was, the higher the valueof the elastic modulus was.

TABLE 7 Stress (distortion factor: Composition [% by weight] 80%) [Pa]Comparative LAPONITE RD 5%/ASAP 1% 7656 Example 1 Comparative LAPONITERD 10%/ASAP 1% 3628 Example 2 Comparative LAPONITE RD 15%/ASAP 1% 5662Example 3 Example 1 LAPONITE RD 5%/ASAP 1%/TSPP 0.5% 76635 Example 2LAPONITE RD 7.5%/ASAP 1%/TSPP 0.5% 77944 Example 3 LAPONITE RD 10%/ASAP1%/TSPP 0.5% 92289 Example 4 LAPONITE RD 12.5%/ASAP 1%/TSPP 0.5% 78459Example 5 LAPONITE RD 15%/ASAP 1%/TSPP 0.5% 133814

Example 30 Uniaxial Compression Test of Hydrogel Using ASAP at EachConcentration

The stress change of each of the hydrogels produced in Example 6 toExample 12 was measured according to Example 29. The results of themeasured stress changes are listed in FIG. 5. A relation between theASAP concentration and the elastic modulus is shown in FIG. 6. As theresult thereof, when the ASAP concentration was 1% by weight, thehydrogel exhibited the highest value of the elastic modulus.

Example 31 Uniaxial Compression Test of Hydrogel Using TSPP at EachConcentration

The elastic modulus of each of the hydrogels produced in Example 13 toExample 24 and Comparative Example 1 to Comparative Example 3 wasmeasured according to Example 29.

The results of Example 13 to Example 15 and Comparative Example 1 areshown in FIG. 7; the results of Example 16 to Example 19 and ComparativeExample 2 are shown in FIG. 8; and the results of Example 20 to Example24 and Comparative Example 3 are shown in FIG. 9. FIG. 10 shows ahydrogel of the D composition and a hydrogel of the E composition inFIG. 9 after the uniaxial compression test. As the result thereof, thehydrogel exhibited the highest value of the elastic modulus in FIG. 7when the TSPP concentration was 0.25% by weight and in FIG. 8 and FIG. 9when the TSPP concentration was 0.5% by weight. However, according tothe comparison in FIG. 10, while the hydrogel of D collapsed, thehydrogel of E did not collapse. As the result thereof, in FIG. 9, whenthe hydrogel was of the composition E (TSPP concentration: 0.75%), thehydrogel exhibited the highest value of the elastic modulus withoutcollapse of the hydrogel.

Example 32 Uniaxial Compression Test of Hydrogel Using ASAP HavingCorresponding Molecular Weight

The fracture stress and the fracture compression ratio of each of thehydrogels produced in Example 25 to Example 27 were measured by the sameoperation as in Example 29. The results thereof are listed in Table 8.The larger the weight average molecular weight of ASAP was, the higherthe value of the fracture stress was. The hydrogel of Example 25produced using ASAP having a small molecular weight collapsed at acompression ratio of 80% or less, which exhibited that such a hydrogelis weak relative to a distortion.

TABLE 8 Fracture Fracture stress compression ASAP molecular weight [Pa]ratio [%] Example 25 250,000 to 710,000^((remark 2)) 22,000 58 Example26 Weight average molecular 45,000 89 weight 2,000,000 to 3,000,000Example 27 Weight average molecular weight 85,000 81 4,000,000 to5,000,000 ^((remark 2))Degree of polymerization × molar mass of monomerunit (molar mass of monomer unit = 94)

Example 33 Measurement of Small Angle X-Ray Scattering (SAXS)

With respect to each of the hydrogels produced in Example 3 and Example1, the measurement of small angle X-ray scattering (SAXS) underextension was performed using a synchrotron small angle X-ray scatteringapparatus (BL-10C; manufactured by High Energy Accelerator ResearchOrganization). The sample was defoamed using a centrifuge (at 7,000 rpmfor 10 minutes) and was molded into a shape of rectangularparallelepiped having a length of 18 mm, a width of 10 mm, and athickness of 1 mm.

A sample which was uniaxially extended in a horizontal direction wassubjected to the measurement using an imaging plate (R-AXIS) as adetector. The results thereof are listed in FIG. 11. As the extensionratio increases, the scattering pattern transitions to a perpendiculardirection relative to the extension direction. It is considered thatthis is because an edge face of the clay particle was aligned in anextension direction according to the extension of an ASAP polymer and asurface of the clay particle faced to the perpendicular direction. Asthe result thereof, it can be considered that there is an interactionbetween the edge face of the clay particle and ASAP and that apolymer/clay network is constructed.

1. A hydrogel-forming composition, comprising: a polyelectrolyte; and clay particles; wherein the polyelectrolyte has a weight average molecular weight of 250,000 to 10,000,000.
 2. The hydrogel-forming composition according to claim 1, wherein the polyelectrolyte is a polyelectrolyte having an organic acid salt structure or an organic acid anion structure.
 3. The hydrogel-forming composition according to claim 2, wherein the polyelectrolyte is a polyelectrolyte having a carboxylic acid salt structure or a carboxylic acid anion structure.
 4. The hydrogel-forming composition according to claim 3, wherein the polyelectrolyte is a completely neutralized or partially neutralized polyacrylic acid salt.
 5. The hydrogel-forming composition according to claim 4, wherein the polyelectrolyte is a completely neutralized or partially neutralized polyacrylic acid salt having a weight average molecular weight of 1,000,000 to 10,000,000.
 6. The hydrogel-forming composition according to claim 1, wherein each of the clay particles is a water-swelling silicic acid salt particle.
 7. The hydrogel-forming composition according to claim 6, wherein each of the clay particles is a water-swelling silicic acid salt particle selected from the group consisting of smectite, bentonite, vermiculite, and mica.
 8. A hydrogel produced from a hydrogel-forming composition comprising: a polyelectrolyte; clay particles; and a dispersant for the clay particles. 